Transmission system



June 4, 19.29. F, MOHR TRANsMIs s ION SYSTEM Filed May 26, 1926 www. @v Q,

Patented .imv 4, 1929.

UNITED STATES" PATENT oF'FlcE.

FRANKLIN MOHR, EAST ORANGE, NEW JERSEY', A-SSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N'. Y., A CORPORATION OF NEW YORK,

TRANSMISSION SYSTEM.

Application filed Hay 26,

This invention relates to transmission systems, and more especially to systems for the transmission of images from one point to another by the use of electricity.

The object of the invention is to increase the efficiency and to improve the serviceability of transmissionsystems of this general character.

It has been proposed to employ frequency modulation in signal transmission systems such for example as shown and claimed, inl

vthe copending application of M. B. Long,

Serial N o. 110,910, filed May 22, 1926.

In accordance withthe presentinvention, however, there is provided an arrangement employing frequency modulations wherein the current forreproducing the picture is the resultant of the output currents of two demodulators, one-of which currents varies as the frequency and the other of which is independent of frequency. Furthermore, there are provided two demodulators so connected that the output currents due to changes in transmission level `are neutralized,

the output current of one demodulator due to frequency changes only affecting the light valve to reproduce the picture.

The drawing is a diagrammatic showing of a picture transmission system including a transmit-ting and a reproducing station and embodying the features of this invention.

The following are two of the various methods of transmitting signals by means ofcarrier current. One is to employ the signal for the purpose of modulating the amplitude of the carrier wave, and the other 1s to employ the signal current for modulating.

the frequency of the carrier wave. The system disclosed herein is of the second type. A carrier current -is generated at the transmitting station and in a .suitable manner modulated as'to its frequency bymeans of 'a current which represents the tone values 192e.l serial No. 111,725.

varies as the frequency of the received wave, or, in other Words, as the tone values of the picture. It is well known, however, that difficulty is had in maintaining the transmission system uniform at all times. That is to say, the so-called transmission level varies due to conditions which cannot be easily controlled. A variation in the trans- 'mission level means a variation in the amplitude vof the transmitted current, which, unless accounted for, will make itself felt in the reproduced picture.

The present invention comtemplates the use of a receiving equipment in which such variations of transmission levels are neutralized or compensated.

Referring particularly.A to the drawing, there is shown a rotatable and axially movable picture drum 2 mounted on a shaft 1 at the transmitting'station. A filmor transv parency 3 of the picture is wrapped about the drum 2 and scanned by a beam of light lfrom the source 5 concentrated by a lens 6 uponthe surface of the drum 2. Within the drum 2 is located a photo-electric cell 4. The cell 4 isconnected across the terminals of a resistance 8 in series with a' source of photo-electric potential '61fo`r the photoelectric current. The resistance 8 is connccted acrossthe input circuit of a picture amplifier PA. Thea'mplifier PA consists of a thermionic discharge device 7 having a flamentary cathode 10, a plate anode 12, and a control electrode 11. The resistance 8 is connected across the control electrode 11 and the filament 10. A battery 9 is connected so as to maint-ain the electrode 11 at a desired potential.

The output circuit ofthe amplifier PA .includes the electrodes 10 and 12 and the source of space current 13. This circuit is connected across the terminals of a winding 16 of a transformer 14. QIf desirable other stages of amplification of/the'resistance type may `be added.

The transformer 14 'is' apart of an oscillating circuit including the oscillator O.

The oscillator O comprises a thermionic discharge device 21 having a filamentary cathode 22, a plate anode 24, anda control grid 23. The input circuit of the oscillator includes the filament A22, the inductive winding 17, and the grid electrode 23. The outv put circuit of the oscillator O has one branch quency currents from flowing therein. An-

other branch of the output circuit includes the condenser 28 and impedance elements 30,

" be variable.

31 and 32. vStill another branch includes thecondenser 28, i'mpedence element 30, resistance 20, inductive winding 15, and condensers 18 and'19 and variable impedance element 16. This latter branch serves as a feedbackcircuit for causing the device21l to oscillate. The condenser 18 may be variable for the purpose of tuning the circuit to secure the desired frequency.. The condenser 19 restricts the current from the source 13 to the plate circuit of the amplifier PA.

The terminals of the impedance element 32 may be connected to a transmission equalizer TE of any suitable design. The equalizer TE is connected to the transmission line L. The purpose of the equalizer TE is to give a uniform amplitude for the various frequencies. yIf necessary, the transmission line L may passthrough a repeating station RS, at which there is -located a repeating amplifier RA controlled in any well known and' suitable manner by a rectifier R and a gain control arrangement GC.

At the receiving station, the transmission line L is connected to a pair of networks 57 and 58. The network 57 may be of any suitable design and, as illustrated, includesy two series loops 33 and 34 each comprising van inductive element and a condenser. The

network 57 alsoincludes a condensers 35 and 36.

The other network 58 comprises two -resistances 59 and 60, the first of which may pair of shunt The output end of the network 57 is connected across a resistance 41. The terminals of the resistance 41 are connected to the input circuit of a demodulator D1. The demodulator D1 consists of a thermionic discharge device 37 having a filament 38, a plate 40, and a grid electrode 39. The gridy electrode 39 is maintained at any desired potential by means of the battery 47. The output circuit of the demodulator D1 includes the plate electrode 40, the space cur- This circuit is connected across the terminals of a high resistance 51.

The second network 58 is connected to a resistance 46. The,l .resistance 46 is connected across the input circuit of a second 4den'iodulator D2. This demodulator comprises a thermionic discharge vacuum tube A42 having a filament 43, plate 45, and a grid electrode 44. Battery 47 also provides a biasmg potential for the grid 44. In similar manner, .the output circuit of the demodulator l)2 includes the battery 50 and is con- A light valve V of any well known type,

such as the vibrating string valve, has its terminals connected across the output circuits of yhe two demodulators l)1 and D2, as illustrated. The valve V serves to control a beam of light vfrom the source 53, through the lenses 54 and 55, to the surface ofthereproducing drum 56. y

The operation of thesystem will now be described. A transparency of film of the picture to be transmitted is mounted upon the drum 2 and scanned Iby the optical syst'em. The variable current flowing through the resistance 8 causes the potential of the grid electrode 11 of the amplifier PA to vary. Thus, the impedance of the plate circuit of the amplifier is Varied, causing a corresponding variation in the current flowing through the winding 16 of the reactive device14. The device 14 is so designed that the carrier current impressed upon the liney L is one the frequency of which varies in accordance 'with the tone values of the picture.

At the repeating station RS, the carrier current transmitted over the line is repeated and amplified in the well known manner.

At the receiving station, the transmitted carrier current having its frequency modulated is impressed upon the network 57. The characteristic of the network 57 is such that the output current .therefrom varies in amplitude inf proportion to the variation in frequency of the current impressed thereon. In this manner the carrier current has been translated into one having an amplitude which varies in accordance with the tone values of the picture. This variable amplitude current flowing through the resistance 41 causes a corresponding variation in the plotentialof the gridelectrode 39 of the demodulator D, In the well known manner .the demodulator D1 produces in its output circuit a current which varies inY proportion to the variation of the grid potential.' c

The transmitted carrier current is also impressed upon the network 58 includingresistances 59 and 60. The network 58v is independent of frequency, and accordingly the amplitude of the current is not altered due to a change offrequency. The Value of the current flowing through the resistance 46 determines the potential of the grid electrede tt of the dcmodulator D2. The potential ol' this grid electrode in turn determines the amplitude ol' the current flowing in the plate circuit of the den'lodulator D2 For ideal conditions, the amplitude of the current flowing in the transmission line Will not vary. That is to say, the transmission level will remain constant. Under these conditions, the range of potential variation of the grid electrode 44 remains substantially constant, and accordingly the amplitude of the current in the output circuit of the demodulator D2 is substantially constant.

The circuits are so adjusted that the value of the current in the output circuit of the dcmodulat or l)2 is equal to the current which y lows in the output circuit of the dcmodulator D1 for a given arbitrary frequency of the transmitted current. Ordinarily, this arbitrarily selected frequency will be the lowest one in the useful scale of frequencies. ln other words, it will be the frequency which is produced due to an extreme valueA of tone. to be found in the picture transmitted. Since the output currents of the demodulators are equal for an eXtreme value of frequency, they may be made to balance or neutralize each other to give a completely closed or a completely open condition of the light valve V, as desired. For other values of frequency in the useful scale, it .follows that the output current of the demodulator D1 exceeds'that of the demodulator D2 by a corresponding amount and accordingly the resultant current Will determine the operation of the valve V, the amount of operation being a measure of the frequency of the transmitted current, or, in other words, a measure of the tone value of the picture being transmitted.

Referring to the output circuit of the dcrnodulator D1, it Will be noted that for the arbitrary value of frequency mentioned, a current of a given amplitude flows in the circuit 40, P, 51, 1) 50, 38. Due to this current, the potential of the point P is raised to a certain Value with respect to the potential of the point P1. At the same time, a current of equal amplitude is iowing in the circuit 45, P2, 52, P1, 50, 43. Accordingly, the potential of the point P2 with respect to the point P, is the same as that for the point P. Therefore, no current will flow through the circuit including the string of the light valve V. As the frequency increases, however. and the current in the output circuit of D1 correspondingly exceeds that of the current in the output circuit of D2, the potential of point P With respect to point P2 increases. This causes a current to flow through the string of the light valve V for actuating the same to govern the intensity of the beam of light striking the drum 5G. Y

llhile the transmission level remains constant under ideal conditions, it is to be expected that 1n practice the transmission 'level will vary due to conditions not easily controlled. It is to compensate for these variations of amplitude that the demodulator D., is particularly adapted. When a given variation in amplitude occurs, due to a change in transmission level, a corresponding change takes place in the potential of the grid electrode 39 of the demodulator D1. This in turn produces a corresponding change in thc amplitude of the current {iowing in resistance 5l. It may be said, therefore, that the output current of the demodulator Dl is made up of two components, one of which is proportional to the frequency of the transmitted carrier current and the other of which is proportional to the amplitude or to the transmission level.

The change of amplitude or of transmission level also produces a corresponding change in the potential of the grid 44 of the demodulator D2. This results in a change in the current flowing in a resistance 52. These changes, namely that in the amplitude of the current in the resistance 5l and that in the amplitude of the current in the resistance 52, are substantially equal. Therefore, the resultant current iiowing in the light valve V is substantially free from the changes in the transmission level.

It may be noted that both the gain con trol device GC at the repeater station and the apparatus at the terminal station serve for the same general purpose. The gain control device GC may be considered as giving supplemental regulation.

It will `be understood that the picture transmission equipment which has only been illustrated schematically, may be of any well known construction. For a more complete understanding of the details of apparatus of this character, reference is made to the Patent of Horton et al., No. 1,606,227 patented Nov. 9, 1926.

What is claimed is:

1. In an image transmission system, transmitting and receiving stations, means at the transmitting station for generating an alternating carrier current, means for modulating the frequency of said current in accordance with the characteristics of a picture or object, a line for transmitting the modulated carrier from the transmitting station to the receiving station, tvvo demodulators at the receiving station having output circuits and being responsive to the frequency modulated carrier, the curr-ent in one output circuit being proportional to variations in the amplitude of the transmitted current, the current in the other output circuit being proportional both to the amplitude and frequency of the transmitted current, a lightv translating device, means for opposingly connecting the output circuits of station, a demodulator having an output circuit and responsive to the transmitted carrier current to produce in said circuit a current proportional only to the amplitude of the transmitted current, a second demodulator having an output circuit and responsive to the transmitted current to produce in its output circuit a current proportional both to the amplitude and frequency of the transmitted current, a light controlling device differentially connected to both output circuits whereby the current flowing in said valve due to currents in said outputcircuits is proportional only to said frequency, and means controlled by said valve for reproducing the picture.

3. In an image transmission system, a transmitting station having means for generating an alternating carrier current, means for modulating the frequency of said current in accordance with the tone values of a picture or object, a receiving station, means for transmitting the modulated carrier current from the transmitting station to the receiving station, two demodulators having output circuits one responsive to the transmitted current to produce in its output circuit a current having two components, one component proportional to the amplitude and the other proportional to the frequency of the transmitted current, the other demodulator responsive to the transmitted current to produce a current in its output circuit which is proportional to the amplitude but independent of the frequency of the transmitted current, a circuit connected to both of said output circuits for causing the currentv in the second output circuit to neutralize the component of the current of the first output circuit which is proportional to amplitude, and to transmit to a light translating device the resulting current which is proportional to frequency, and a light valve translating device having impressed thereon the said resultingcurrent from the two demodulators.

4. The combination in an image transmission system, of means for generating an oscillatory carrier current, means `for modulating the frequency of said current in accordancewith the characteristics of a picture or object, means for transmitting the modulated current, a network for receiving and translating .the tranismittcd current into a current of varying amplitude, ade# modulator connected to said network 'andl arranged to produce a current in its output circuit proportional both to said frequency and to the amplitude of the modulated carrier current, a second network for receiving y means at the transmitting station for gener-- ating an alternating carrier current, means for modulating the frequency of said current in accordance with the characteristics of -a signal, a line for transmitting the modulated carrier from the transmitting station to the receiving station, two demodulators' at the receiving station having output circuits and being responsive to the frequency modulated I carrier, the current in one outputl circuit being proportional to variations in the amplitude of the transmitted current, the current in the other output circuit being proportional both to the amplitude and frequency of the transmitted current, a signal translating device, means for opposingly connecting the output circuits of the said two demodulators, and means for impressing the resultant output current from the said demodulators upon the said signal translating device.

6. In an electrical signal transmission system, a transmitting station having means for generating an alternating carrier current, means for modulating the frequency of said current in accordance with 4a signal, a receiving station, means for transmitting the modulated carrier current from the transmitting station to the receiving station, two demodulators having output circuits, one responsiveto the transmitted current to produce in its output circuit a current having two components, one component proportional to the amplitude and the other proportional to the frequency of the: transmitted current, the other demodulator responsive to the transmitted current to produce a current in its output circuit which is proportional to the am litude but independent of the frequency of) the transmitted current, a circuit connected to both of said output circuits for causing the current in the second output circuit to neutralize the component of the current of the first output circuit which is proportional to amplitude and to transmit to a signal translating device the resulting current which is proportional to frequency, and a signal translating device having impressed thereon the said resulting current from the two demodulators.

7. A signal transmission system comprising means for producing a carrier Wave the frequency of which is modulated in accordance with the signal, a transmitting medium in which variations in attenuation may take place, means for receiving the modulated carrier Wave comprising two paths in parallel relation with respect to the incoming carrier wave which paths are substantially equally responsive to amplitude variations in said wave, a signal reproducing device in balanced relation with respect to said paths and frequency discriminatmg means in one of said paths for unbalanemg said paths with respect to said si al reproducing device for frequency varlations of said wave.

8. A signal transmission system in accordance with claim 1 comprising detecting means for detecting said high frequency wave to produce a low frequency component for operating said signal reproducing device. In witness whereof, I hereunto subscribe my name this 22 day of May, A. D.y 1926.

FRANKLIN MOI-IR. 

